/Engine/Source/Runtime/RHI/Public/GPUDefragAllocator.h
Performs a partial defrag by shifting down memory to fill holes, in a brute-force manner. Takes consideration to async reallocations, but processes the all memory in order.
void PartialDefragmentationDownshift ( [FRelocationStats](API\Runtime\RHI\FGPUDefragAllocator\FRelocationStats) & Stats, double StartTime )